Mechanical failure of optical fibers under stress over extended periods of use is a phenomenon known as static fatigue. It is well known that bare uncoated fibers are susceptible to abrasion which results in surface scratches. These scratches can produce a significant loss of light through the sides of the fiber and can also result in sudden failure through breakage of the fiber. Optical fibers are susceptible to breakage not only because they are formed from relatively brittle materials, but in addition, the fibers typically have very small diameters and are subjected to an assortment of stresses such as bending stresses, tensile stresses, and the like.
Optical fibers are also susceptible to stress corrosion cracking. Water or hydroxyl ions can react with a fiber under stress, thus damaging its optical properties and weakening its mechanical strength and static fatigue resistance. Microcracks in fiber surfaces present regions which are susceptible to attack by hydroxyl ions or moisture when the fiber is under stress. Such stress tends to open these cracks thereby straining the chemical bonds at the tips of the cracks. These strained bonds are readily attacked by moisture enabling the phenomenon called stress corrosion cracking to proceed and propagate such microcracks. Growth of these microcracks weakens the fiber continuously over a period of time until it produces sudden failure. As mentioned above, this problem is termed static fatigue.
One solution to the problem of static fatigue is to put an impervious hermetic coating around the fiber, so that the atmospheric moisture cannot reach the fiber surface. Various types of coatings have been investigated. A metallic seal of aluminum has been suggested as a hermetic coating ("Reduction in static fatigue of silica fibers by hermetic jacketing"--Pinnow, Robertson, Wysoski--Appl. Phys. Lett. 34(1), January 1979), however, metals tend to form polycrystalline solids which can themselves be corroded by moisture or by enhanced grain boundary diffusion. Metal coatings also provide undesirable electrical paths along the fiber.
Several non-metallic coatings have also been utilized. For example, silicon nitride (U.S. Pat. No. 4,028,080 to DiVita et al) has been investigated as a potential coating, but silicon nitride has been seen to weaken the fiber substantially due to residual stress in the coating. Also, it is difficult to make strong fibers in long lengths with silicon nitride. Pyrolytic carbon has also been suggested in U.S. Pat. No. 4,183,621 to Kao et al and sputter deposited carbon was suggested by Stein et al ("Ion plasma deposition of carbon-indium hermetic coatings for optical fibers"--Proceedings of Conference of Laser and Electro-Optics, Washington, D.C., June 10-12, 1982). But in both cases, the crack velocity exponent N of the coating were determined to be in the range of 23-30. This means that such coatings are really not hermetic. It has recently been reported that silicon carbide is a good hermetic coating (U.S. Pat. No. 4,512,629 to Hanson et al). The results of investigation with silicon carbide indicate a N value of 100 or higher can be obtained with good median strength.